1. Field of Invention
The present invention relates to a motor driving circuit, and in particular to a motor driving circuit capable of providing stable currents or rotation speeds to allow the motor operates stably.
2. Related Art
With the technological progress and development of the computer industry, a portable electronic product such as a notebook computer has gradually become a mainstream in the market. In the portable electronic products, the heat dissipating ability always influences the stability, the performance and the lifetime of the product. For example, a computer system is usually equipped with a fan serving as a heat-dissipating device in order to quickly dissipate the heats generated by the computer system. Thus, the computer system may work under a suitable temperature environment.
Generally speaking, most fans for dissipating heats in the computer systems are driven by brushless DC motors. As shown in FIG. 1, a client system end 6 transmits a pulse width modulation (PWM) signal SPWM to a DC driving circuit 4 so as to drive a motor 5 to rotate by open loop. FIGS. 2A to 2C show motor characteristic curves corresponding to the current, the rotation speed and the air pressure of the conventional motor driving circuit. When the motor 5 works with multiple voltage, the current or rotation speed may be unstable due to different shapes of blades of the motor 5. If the unstable current gets high, electronic elements may be damaged. If the current gets low, the element availability may be wasted. The unstable rotation speed complicates the control of the motor, and the air pressure thereof is also decreased.